Field of the Invention
This invention relates to the removal of contaminants from water discharged at a gas wellhead, and more particularly to the removal of contaminants from fracture water that is used to initially used to fracture deep subterranean rock formations near a well bore, and that is subsequently discharged from the wellhead.
Description of Related Art
Certain subterranean geologic formations contain significant reservoirs of natural gas. For example, the Marcellus Shale subterranean formation, which extends from central and western New York southwardly through Pennsylvania, West Virginia, and eastern Ohio, contains significant natural gas deposits. Extraction of this gas has been made economically and technically feasible by the utilization of a technology referred to as “hydrofracing.” This technology utilizes injection of large volumes of water at high pressure to fracture the subterranean shale structures, which causes them to separate and release the pockets of methane gas contained within the shale strata.
This injected water returns initially to the surface as initial “flowback water” and over the life of the well as “production water.” The return water contains contaminants from the subterranean structures, as well as from pretreatment of the water with certain chemicals prior to performing the hydrofracing injection. As much as one million gallons or more of flowback water may be generated within the first weeks following a typical fracturing injection, and an additional two million to three million gallons of production water may be produced from the well over the life of the well's methane production cycle of multiple years.
The contaminants in this water render it unsuitable for reuse except in small proportions. Disposal of the water has become an expensive proposition for the well developers and also a difficult environmental problem. New environmental regulations, as well those proposed and likely to be implemented, will essentially eliminate the ability to dispose of or treat this water by conventional means within the states of Pennsylvania and New York. There is a significant need by the well developers for technologies which will treat the water to enable its reuse to a maximum extent.
The contaminants in the water include many of the following components.                Sodium Chloride        Barium Salts—typically in the chloride form        Strontium Salts—typically in the chloride form        Calcium and Magnesium (Hardness) Salts—typically in the bicarbonate and/or chloride form        Volatile Organic Compounds (VOC) resulting from the degradation of the agent sea creatures captured within the shale formation        Residual Organic Compounds (ROC) from the water treatment chemicals introduced into the fracing water to enhance the hydrofracing process        Iron Salts—in multiple species        
The sodium chloride is not considered a significant problem relative to the reuse of the water up to some practical limit that is determined by the hydrofracing operator. The barium and strontium salts form insoluble solids during the fracing process which have been determined to be degradative to the mining equipment due to abrasion when the water is reused and the concentrations of these materials exceed 100 ppm. The calcium salts form hardness scale in the subterranean shale structures and are similarly limited in concentration but at much higher levels. Both of these conditions may result in the obstruction of portions of the fissures within the shale strata created by the fracing process when the water is reused, and the concentrations of these salts are excessive. The presence of the residual or incipient volatile organic chemicals in the return water results in further difficulties in proper reformulation of it as fracing water for reuse.
The chemistry of the removal of the barium and strontium salts and the calcium salts is well defined in various documentation and literature. The solidification of all of the salts may be accomplished by conventional evaporation technology. However, the contamination of the solid sodium chloride salt by the calcium, and in particular the barium and strontium, make its disposal difficult and its beneficial reuse not feasible. The current method of return water remediation is to transfer it upon its discharge from the wellhead into tanker trucks, and then haul it substantial distances to off-site water treatment plants. This is expensive, and additionally, it will be prohibited or severely limited in many jurisdictions (particularly New York and Pennsylvania) by new environmental regulations taking effect in early 2011. Semi-portable processing systems are also in use on a limited basis in operations in this formation. However, recently promulgated regulations now limit the space available at well site operations and remote treatment operations. This severely limits the ability to treat the flowback water at a well site. Furthermore, the well site conditions require rapid treatment of the flowback water. Conventional systems which may be portable (or semi-portable) require a considerable footprint in order to meet the flow requirements.
In addition, the flowback water from the hydrofracturing of certain sections of the Marcellus Shale formation contains levels of barium and strontium contamination significantly higher than previously encountered in other formations. To the best of the Applicant's knowledge, treatment of these elevated levels of barium and strontium in a cost effective and portable format has not yet been achievable.
What is needed is a modular, portable and cost effective method and apparatus for treating the frac water for the removal of the barium, calcium and strontium salts onsite at the wellhead.